1. Field of the Invention
The present invention relates to an elastic wave device using plate waves, and more specifically relates to an elastic wave device including a LiTaO3 substrate.
2. Description of the Related Art
SH wave devices using LiTaO3 are conventionally provided. For example, Japanese Unexamined Patent Application Publication No. 2004-260381 described below discloses an SH wave device using an SH0 mode, that is, a basic mode of SH waves. Japanese Unexamined Patent Application Publication No. 2004-260381 discloses that the use of a LiTaO3 substrate of a predetermined thickness brings about a temperature coefficient of delay TCD of approximately 0. It is also described that in the case of an SH wave device used in a 10 MHz band, forming an electrode material film of gold or aluminum into a film thickness of the order of 0.2 μm brings about a TCD of approximately 0.
When normalizing the film thickness of the electrode film described in Japanese Unexamined Patent Application Publication No. 2004-260381 by a wavelength determined by the pitch of electrode fingers of the SH wave device, the Al equivalent wavelength-normalized thickness of an IDT electrode becomes extremely thin i.e. 0.000625λ. That is to say, the electrode film must be formed extremely thin, i.e., 0.1% or less of the wavelength. In the case of use at a frequency of the order of 10 MHz, just as with an embodiment of the Japanese Unexamined Patent Application Publication No. 2004-260381, the film thickness of the electrode film is not so thin, i.e., on the order of 0.2 μm. However, the film thickness of the electrode film becomes 4 nm in the case of use at a high frequency band of 500 MHz, and the film thickness of the electrode film becomes approximately 2 nm in the case of use in a 1 GHz band. Thus, the resistance of electrode fingers significantly increases with a decrease in the film thickness of the electrode film. This results in an increase in loss. In addition, when the film thickness of the electrode film becomes too thin, the electrode fingers of a minute pitch are very difficult to form with high precision.